| Modular multilevel converter(MMC)has become one of the mainstream topologies in high voltage and high power HVDC transmission system,because of its outstanding technical advantages such as high output waveform quality,high efficiency and high reliability.However,related studies and reports have shown that MMC,as a key power conversion device,may oscillate when it is connected to grid,which hinders the normal transmission of electric energy.At present,many researches expect to study the mechanism of MMC oscillation by impedance-based analysis method,but the existing impedance models ignore the complex harmonic characteristics caused by the structure of MMC,and have not taken DC voltage and power control structure into consideration,which may make the impedance characteristics greatly different from the actual system,resulting in inaccurate analysis results.Therefore,accurate extraction of MMC impedance characteristics has become one of the keys to study the performance of MMC system.Aiming at grid connected MMC in HVDC system,the harmonic characteristics of grid connected MMC are analyzed based on the average mathematical model,which lays the foundation for the frequency domain modeling and analysis process.The accuracy of the average model is verified by simulation analysis in the time domain.After applying average model,the simulation analysis efficiency is effectively improved.After that,based on the multi-harmonic linearization method,the main circuit model of grid connected MMC is established in the matrix form in the frequency domain,considering the harmonic characteristics.The typical double closed loop control structure is analyzed in the frequency domain,and the relationship between the inner loop and the outer loop with asymmetric structure is clarified,then the model of double closed loop control structure is established.Meanwhile,the model of circulating current suppression structure and the phase-locked loop are deduced.Then a more accurate AC and DC side impedance model of grid connected MMC is established,which makes the impedance analysis results more accurate.Finally,the impedance model is verified by simulation results and the main factors affecting the impedance characteristics of grid connected MMC are analyzed,and the impacts of each factor on the system oscillation are discussed.Moreover,the oscillation cases of grid connected MMC when connected to non-ideal grid is analyzed by impedance analysis method,which verifies the application value of the established impedance model. |
PDF Full Text Request